Heterojunction Bipolar Transistors (HBTs) are known in the art. For example see U.S. Pat. No. 4,768,074 entitled, "Heterojunction Bipolar Transistor Having an Emitter Region with a Band Gap Greater than that of a Base Region" to Yoshida et al. Double Heterojunction Bipolar Transistors (DHBTs) and Double Heterojunction High Electron Transistors (DHETs) are known in the art. For example see U.S. Pat. No. 5,010,382 entitled "Heterojunction Bipolar Transistor Having Double Hetero Structure" to Katoh. DHBTs and DHETs (DHTs) have one heterojunction between the emitter and base region and a second between the base and collector region. DHTs have many advantages over other types of bipolar transistors, such as enhanced emitter injection efficiency, lower base resistance, and lower base-emitter junction capacitance (C.sub.jbe).
FIG. 1 is a cross-sectional view of a prior art GaAs-AlGaAs HBT 100 structure. The HBT 100 has an n-type GaAs collector layer 102, a p-type GaAs base layer 104, and an n-type Al.sub.x Ga.sub.1 -x As emitter 106 (x is the mole fraction of aluminum in AlGaAs) on an n.sup.+ -GaAs substrate 108. The emitter 106 has two layers, a thick n.sup.- -type first emitter layer 106a on the base layer 104 and a thin n.sup.+ -type second emitter layer 106b on the first emitter layer 106a and contacting emitter electrode 110. Collector electrode 112 contacts the sub-collector layer 108 and base electrode 114 contacts base layer 104.
The lower doping of first emitter layer 106a combined with its thickness reduces C.sub.jeb and increases the transistor's switching speed. To further improve transistor performance, the emitter and the collector current density must be at least 10.sup.3 to 1.0.sup.4 Amp/cm.sup.2. For the prior art HBT of FIG. 1, the reduced doping concentration of the emitter layer 106a reduces carrier injection into the base from the emitter to slow transistor turn-on. Because of this low doping concentration, a high forward-bias voltage V.sub.be is applied to the base-emitter junction to increase current density. However, because of this increased V.sub.be, excess carriers are stored in both the first emitter layer 106a and in collector layer 102. Consequently, the transistor's turn-off time t.sub.off increases. Since transistor switching speed is the average of t.sub.on and t.sub.off, a large t.sub.off offsets a reduction in t.sub.on and, therefore, is unacceptable.
A heterojunction formed from dissimilar semiconductor materials causes a conduction band discontinuity or spike, .DELTA.E.sub.c, and a valence band discontinuity .DELTA.E.sub.v at the interface of the two materials. .DELTA.E.sub.c blocks the injection of low-energy carriers from the emitter region into the base region degrading emitter efficiency and, consequently, switching speed. Prior art attempts, e.g., grading the heterojunction, have failed to solve this switching speed degradation problem. For example, see L. F. Eastman, P. M. Enquist, and L. P. Ramberg, "Comparison of Compositionally Graded to Abrupt Emitter-Base Junctions Used in Heterojunction Bipolar Transistor," Journal of Applied Physics, Volume 61, pps. 2663-2669, 1987.
Prior art HBTs and DHBTs also suffer from high junction leakage currents. Several factors contribute to junction leakage, including base electron-hole recombination and laterally diffused carriers-injected from the emitter into the extrinsic base. Consequently, HBTs have a lower current gain B than would otherwise be expected. Exacerbating this problem is the HBT .beta.'s non-uniformity, and the further reduction of .beta. that results when HBT's are scaled. These problems compound each other, making HBTs unattractive for dense circuit integration.